This invention relates to an apparatus and process for preparing calcined gypsum whose properties in general, e.g., set time, pouring consistency, rate of strength development and other properties, are particularly adapted for use in gypsum board manufacture. The invention also relates to a process for continuously treating calcined gypsum to produce a healed calcium sulfate hemihydrate having properties of low water demand, which provides significant energy savings, yet retaining other properties required for use in gypsum board production.
Continuous calcination of gypsum, such as disclosed in U.S. Pat. No. 3,236,509, is of importance in the economical production of gypsum stucco (calcium sulfate hemihydrate) especially adapted for use in gypsum board manufacture. The properties required of gypsum stucco used in gypsum board manufacture on modern automatic machinery differ substantially from the properties required for so-called bag plaster usually made in kettles operated by batch techniques. It has been found that stucco produced by the continuous process differs principally from batch process stucco in aging and setting properties. Because the continuously produced stucco is not going to be stored for any appreciable period of time, but rather is going to be utilized immediately or shortly after calcination in gypsum board manufacture, the gypsum material does not need to exhibit long-term stability.
In automated gypsum board manufacture a large portion of the processing time and processing energy is devoted to removing excess water from the wet board. Considerable excess water is required in gypsum board manufacture to properly fluidize the calcined gypsum and obtain proper flow of the gypsum slurry. Thus, calcined gypsum made by continuous calcination will have a dispersed consistency of about 120-200 cc. This relates to a water usage of 85-100 parts of water per 100 parts of the calcined gypsum in a modern automated gypsum board plant. The theoretical water required to convert the calcined gypsum (calcium sulfate hemihydrate) to set gypsum dihydrate is only 18.7% by weight on a pure basis. This leaves about 67% to about 82% of the water present in the gypsum slurry which must be removed in drying the board.
For a considerable time the art has sought to reduce the cost and increase the line speed in the manufacture of gypsum board. This could be accomplished, if effective practical means could be found, by reducing the amount of water required to properly fluidize the calcined gypsum supplied to the automatic board production machinery. That is, if the water demand required for dispersed consistency could be reduced approximately 20-50%, then there would be corresponding less free water to be removed from the wet slurry of the board. This effectively would lower the energy required in drying a given volume of wet board and allow either less time in the drying ovens, lower temperature to effectuate complete drying in the ovens, or both.
Methods of producing artificially aged or low consistency kettle stucco have been accomplished using batch kettle operations. Thus, the so-called "aridizing process" disclosed by Brookby in U.S. Pat. No. 1,371,581 and subsequent patents related thereto have disclosed the addition of a deliquescent substance such as a chloride of an alkaline earth metal into the stucco during the batch kettle calcination. However, this method is not effective with continuously calcined gypsum. Further, the addition of salt is undesirable for the reason that this hygroscopic impurity is very deleterious to board qualities such as plastic flow and bond of paper to core.
It has also been long recognized in the art of batch calcination that artificial aging could be accomplished with water addition. One such attempt is described in U.S. Pat. No. 1,713,879 in which either "single boil" or "double boil" calcined gypsum was aged by placing one ton of stucco in a plaster mixing machine and adding water over a five to six minute period. Another such attempt is described in U.S. Pat. No. 2,177,668 in which calcined gypsum having a combined moisture content of about 2% (showing the presence of much active anhydrite) was exposed to a humidified gas such as air at 60% relative humidity until sufficient moisture was supplied to the over-calcined gypsum to rehydrate the anhydrite content to the hemihydrate The calcined gypsum so treated by these processes not only showed a reduction in consistency but also set and hardened more quickly. Thus, for example, a water quenching step which reduced the consistency of the quenched calcined gypsum to a value of 56 immediately shortened the set time to 25 minutes and after aging 14 days exhibited a set of only six minutes.
Obviously, such quick setting material is not suitable, without excessive corrective reformulation, for automated continuous board production, since the slurry mass would begin setting during mixing in the board slurry mixer, thus resulting in poor quality board and considerable equipment shutdown and clean-up. Kinkade and McCleary in U.S. Pat. No. 3,415,910 taught that the quick set deficiency of batch quenched calcined gypsum could be overcome by quenching the calcined gypsum while the mass of calcined gypsum is still hot with considerably large quantities of water to quench the stucco and then stabilizing the material by reheating the kettle contents to about 255.degree. F. This stabilized stucco, so treated, not only showed a reduction in consistency but also showed that the setting time and dispersed surface area did not vary excessively upon aging. Such treatment requires considerable water in cooling down the "hot" stucco and also considerable utilization of energy in both initially heating the stucco before quench and also reheating the quenched stucco to stabilize it. The longer cycle time per batch for the two additional operations would require added capital investment to supply automated board lines and is energy intensive. Further, this process reduces the effective capacity of the kettles by at least 50% and would be economically unfeasible for board manufacture at present fuel and added equipment costs.
One skilled in the art, in view of the above patent literature, would expect that water-treated calcined gypsum would be subject to quick set and would cause immediate setup in processing lines unless stabilized as taught in Kinkade et al. Attempts to adapt the water treatment alone to continuous calcination resulted in products exhibiting incomplete treatment, shortening of set time, and high energy consumption. Thus, while these treatments have been employed for batch operations, it has been commonly recognized that they are not suitable for continuously calcined gypsum for utilization in gypsum board manufacture under automated processes.